Solution for lithium secondary batery and lithium secondary battery including same

ABSTRACT

Proposed is an additive for an electrolyte solution to improve the electrochemical properties of a lithium secondary. 
     Especially, an electrolyte for a lithium secondary battery includes: a lithium salt; a solvent; and a functional additive, wherein the functional additive includes 1,2-bis(maleimido)ethane, represented by the following formula 1: 
     The electrolyte solution for a lithium secondary battery includes: an electrolyte salt and an organic solvent, wherein the electrolyte solution further comprises, as additives, vinylene carbonate (VC) represented by the following Formula 1, and a compound represented by the following Formula 2, (5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile):

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2021-0191206, filed Dec. 29, 2021, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND 1. Field

The present disclosure relates to an electrolyte solution constituting a lithium secondary battery and to a lithium secondary battery including the same. Specifically, the present disclosure relates to an additive for an electrolyte solution to improve the electrochemical properties of a lithium secondary battery.

2. Description of the Related Art

Batteries are energy storage sources capable of converting chemical energy into electrical energy or electrical energy into chemical energy. Batteries can be divided into non-reusable primary batteries and reusable secondary batteries. Compared to primary batteries which are used once and discarded, secondary batteries are eco-friendly because they can be reused.

Recently, as environmental problems have emerged, the demand for hybrid electric vehicle (HEV) and electric vehicle (EV) with little or no air pollution is increasing. In particular, EVs are vehicles in which the internal combustion engine has been completely removed, suggesting the future direction the world should take.

A lithium secondary battery is used as an energy source for EVs. A lithium secondary battery is largely composed of a cathode, an anode, an electrolyte, and a separator. In the cathode and anode, intercalation and deintercalation of lithium ions are repeated to generate energy, an electrolyte becomes a path for lithium ions to move, and in the separator, the cathode and anode meet to prevent a short circuit in a battery.

In particular, the cathode is closely related to the capacity of the battery, and the anode is closely related to the performance of the battery, such as high-speed charging and discharging.

The electrolyte is composed of a solvent, an additive, and a lithium salt. The solvent becomes a transport channel that helps lithium ions move back and forth between the cathode and the anode. In order for a battery to have good performance, lithium ions must be rapidly transferred between the cathode and the anode. Therefore, selecting an optimal electrolyte is very important in order to obtain excellent battery performance.

In particular, a thin film called solid electrolyte interphase (SEI) is formed on the anode in the chemical conversion process performed during the production process of the battery. SEI is a membrane that can pass lithium ions but not electrons and prevents battery performance from degrading because electrons pass through SEI and induce additional reactions. In addition, the SEI suppresses the direct reaction of the electrolyte and the anode and suppresses the separation of the anode.

The additive of the electrolyte is a substance added in a trace amounts of 0.1 to 10% with respect to the weight of the electrolyte. Despite the trace amount added, the performance and stability of the battery are greatly affected by the additives. In particular, the additive induces the formation of SEI on the surface of the anode and plays a role in controlling the thickness of the SEI. In addition, the additive can prevent the battery from being overcharged and can increase the conductivity of lithium ions in the electrolyte.

On the other hand, the energy density of lithium secondary batteries largely depends on the characteristics of cathode and anode materials, and it is necessary to develop a suitable electrolyte for the developed cathode and anode materials to exhibit excellent electrochemical performance.

Recently, in NCM-based oxide, which is a high-capacity cathode active material, the cathode capacity may be increased by increasing the Ni content or the high voltage of the charging voltage, but the residual lithium (Li₂CO₃ and LiOH) components on the surface of the cathode accelerate electrolyte decomposition and also increase a degradation rate due to an increase in interface reactivity with the electrolyte, thereby degrading a lithium secondary battery and rapidly degrading electrochemical performance.

Therefore, it is necessary to introduce an additive capable of forming an electrochemically and chemically stable SEI.

The matters described as the background art above are only for improving the understanding of the background of the present disclosure and should not be accepted as acknowledging that they correspond to the related art already known to those of ordinary skilled in the art.

BACKGROUND OF THE DISCLOSURE

The present disclosure is proposed to solve these problems, and an objective of the present disclosure is to provide an electrolyte solution additive capable of improving the electrochemical properties of a lithium secondary battery by being added to the electrolyte solution of a lithium secondary battery.

An electrolyte solution for a lithium secondary battery including an electrolyte salt and an organic solvent, the electrolyte solution includes vinylene carbonate (VC) represented by the following Formula (1), and further including a compound represented by the following Formula (2), (5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile), as an additive:

5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile may be included in an amount of 0.1 to 2% by weight with respect to the total weight of the electrolyte.

VC may be included in an amount of 0.1 to 10% by weight with respect to the total weight of the electrolyte.

The addition ratio of 5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile to VC may be 1:10.

Electrolyte salts may be mixed with any one or two or more compounds selected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiCl, LiBr, LiI, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF_(3.0)CO₂, Li(CF₃SO₂)_(3.0)C, LiAsF₆, LiSbF₆, LiAlCl₄, LiCH₃SO₃, LiCF₃SO₃, LiN(SO_(2.0)C₂F₅)₂, Li(CFSO₂)₂N, LiC₄F₉SO₃, LiB(C₆H₅)₄, and Li(SO₂F)₂N(LiFSI).

The electrolyte salt may be included in a concentration of 0.5 M to 1.0 M.

The organic solvent may be any one or two or more solvents selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

The lithium secondary battery, including the electrolyte solution, includes a cathode, an anode, and a separator interposed between the cathode and the anode, and the cathode of the lithium secondary battery may include a nickel-cobalt-manganese (NCM)-based cathode active material, at this time, nickel, cobalt, and manganese may have a ratio of 6:2:2 to 8:1:1.

When the electrolyte solution including 5-(4-nitrophenyl)-1-(3-(trimethylsilyl)oxy)propyl)-1H-1, 2, 3-triazole-4-carbonitrile in accordance with this disclosure is used for a lithium secondary battery, a lithium secondary battery with improved electrochemical characteristics may be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the energy levels of HOMO and LUMO of 5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile;

FIG. 2 is a graph showing the initial efficiency for Comparative Examples and Examples;

FIG. 3 is a graph showing the high-temperature life for Comparative Examples and Examples; and

FIG. 4 is a graph showing high-rate characteristics for Comparative Examples and Examples.

DETAILED DESCRIPTION

Hereinafter, specific contents for solving the above-described objective and problems will be described in detail with reference to the accompanying drawings. On the other hand, when the detailed description of a known technology in the same field is not helpful in understanding the core content of the disclosure in understanding the present disclosure, the description will be omitted, and the technical spirit of the present disclosure is not limited thereto and may be variously implemented by being changed by those skilled in the art.

One feature of the present disclosure is that the electrochemical properties of a lithium secondary battery may be increased by using vinylene carbonate (VC) represented by Formula 1 as an additive and 5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile represented by Formula 2 as an additive, simultaneously.

Formulae 1 and 2 are as follows.

Hereinafter, VC is represented as additive 1, and 5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile is represented as additive 2.

By using additive 1 and additive 2 simultaneously, cathode electrolyte interphase (CEI) is formed on the cathode, and SEI is formed on the anode, thereby improving the lifespan and output characteristics of the battery.

Specifically, since additive 2 has a low LUMO energy level and a high HOMO energy level compared to other materials included in the electrolyte, it is expected to react first on the cathode and anode surfaces to form CEI and SEI.

FIG. 1 shows the energy levels corresponding to LUMO and HOMO of additive 2, and CN10 corresponds to additive 2 of the present disclosure. Referring to FIG. 1 , the HOMO energy level of additive 2 is −6.37 eV, and the energy level of LUMO is −4.15 eV, and it may be expected that the reaction will occur as it is easily decomposed at the cathode and the anode than EC and FEC used as solvents, VC used as additives, and LiPO₂F₂.

In particular, it is possible to form a strong film on the cathode and the anode with respect to triazole, which is the center of additive 2. Triazole-based additives are expected to play a similar role because they have a structure similar to those of the imidazole structure. In addition, since the unshared electron pair present in the sp2 orbital of the nitrogen atom of the triazole structure has characteristics that are not included in the electron cloud, the additives can stabilize moisture, acid, and PF₅ and suppress further decomposition of the LiPF₆ salt.

In addition, degradation of the battery can be suppressed by removing HF acting as a degradation factor of the battery through the TMSO functional group contained in the additive agent 2.

Finally, a transition metal such as nickel may be stabilized through a nitrile functional group. The nitrile functional group has a structure in which carbon and nitrogen are bonded by a triple bond, so the force to attract electrons of the unshared electron pair existing in the nitrogen atom is stronger and coordinates with the metal (metal-coordination complex). In the case of an NCM-based cathode used as a cathode active material for a lithium secondary battery, when the nickel content approaches 80%, structural stability decreases as charging and discharging proceed, causing a cation mixing problem in which nickel moves to the lithium layer alternatively, the nickel layer present on the surface receives electrons from the surface of the cathode, is oxidized, and eluted into nickel ions, which move back to the surface of the anode to form a nickel fluoride layer and act as an element hindering ion transfer.

In the present additive, a function of preventing the dissolution of the transition metal from the cathode and consequently preventing electrodeposition to the anode is expected due to the high affinity between a transition metal such as nickel and a nitrile functional group.

Hereinafter, the results of experiments on electrochemical properties by manufacturing a lithium secondary battery using the additive will be described.

The cathode includes an NCM-based cathode active material made of Ni, Co, and Mn, and in particular, NCM811 was used in this embodiment. As the cathode active material may be used of LiCoO₂, LiMnO₂, LiNiO₂, LiNi_(1-x)Co_(x)O₂, LiNi_(0.5)Mn_(0.5)O₂, LiMn_(2-x)M_(x)O₄ (M is Al, Li or a transition metal), LiFePO, and the like, and all other cathode active materials that can be used for lithium secondary batteries may be used.

The cathode may further include a conductive material and a binder.

The conductive material is used to impart conductivity to an electrode. Any electronically conductive material without causing chemical changes in a configured battery can be used as a conductive material. For example, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, metal powder such as copper, nickel, aluminum, silver, metal fiber, and the like can be used as a conductive material, and one type or a mixture of one or more types of conductive materials including polyphenylene derivatives may be used.

The binder serves to adhere the particles of the active material well to each other or to the current collector, which is to mechanically stabilize the electrode. That is, the active material is stably fixed in the process of repeated intercalation and deintercalation of lithium ions to prevent the loosening of the bond between the active material and the conductive material. The binder may include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like, but is not limited thereto.

The anode includes any one or more of carbon (C)-based or silicon (Si)-based anode active material, and the carbon-based anode material may include one or more materials selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, graphitized meso-carbon microbeads, fullerene, and amorphous carbon, and the silicon-based anode active material may include any one of SiO_(x) and silicon-carbon composite materials. In particular, a graphite anode active material was used in this embodiment.

Like the cathode, the anode may further include a binder and a conductive material.

The electrolyte solution is composed of an organic solvent and additives.

The organic solvent may include any one or two or more solvents selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

The carbonate-based solvent may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), and the like. In addition, γ-butyrolactone (GBL), n-methyl acetate, n-ethyl acetate, n-propyl acetate, and the like may be used as the ester-based solvent, and dibutyl ether may be used as the ether-based solvent but is not limited thereto.

The solvent may further include an aromatic hydrocarbon-based organic solvent. Specific examples of the aromatic hydrocarbon-based organic solvent may include alone or in a combination of benzene, fluorobenzene, bromobenzene, chlorobenzene, cyclohexylbenzene, isopropyl benzene, n-butylbenzene, octyl benzene, toluene, xylene, mesitylene, and the like.

The separator prevents a short circuit between the cathode and anode and provides a passage for lithium ions to move. Such separators may include known materials such as polyolefin-based polymer membranes such as polypropylene, polyethylene, polyethylene/polypropylene, polyethylene/polypropylene/polyethylene, and polypropylene/polyethylene/polypropylene, or multilayers thereof, microporous films, woven fabrics, and non-woven fabrics. In addition, a film coated with a resin having excellent stability on the porous polyolefin film may be used.

Preparation and experimentation of batteries corresponding to Comparative Examples and Examples

<Preparation of Cathode>

For the preparation of the cathode, PVdF was dissolved in NMP to prepare a binder solution.

A slurry was prepared by mixing the cathode active material, and Ketjen Black used as a conductive material in a binder solution. The slurry was coated on both sides of an aluminum foil and dried.

After that, a rolling process and a drying process were performed, and the aluminum electrode was ultrasonically welded to prepare a cathode. In the rolling process, the thickness was adjusted to be 120 μm to 150 μm.

In this case, Li[Ni_(1-x-y)Co_(x)Mn_(y)]O₂ (1-x-y≥0.6), a material in which Ni, Co, and Mn were mixed in the ratio of an 8:1:1, was used as the cathode active material.

<Preparation of Anode>

A slurry was prepared by mixing the anode active material, and Ketjen Black used as a conductive material in a binder solution. The slurry was coated on both sides of an aluminum foil and dried.

After that, a rolling process and a drying process were performed, and the aluminum electrode was ultrasonically welded to prepare a cathode. In the rolling process, the thickness was adjusted to be 120 μm to 150 μm.

At this time, graphite was used as the anode active material.

<Preparation of Electrolyte Solution>

A mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 25:45:30 was used as an organic solvent, and 0.5 M LiPF₆ and 0.5 M LiFSI were dissolved in the solvent as lithium salts, and the electrolyte was injected. In addition, according to each Example, different ratios of additive 2 were added to the organic solvent.

<Preparation of Coin Cell>

After interposing a separator between the cathode and the anode, and then wound to prepare a jelly roll. A coin cell was prepared using the prepared jelly roll and electrolyte.

Comparative Example 1

Only additive 1 (1.0% by weight) was used as an additive in the electrolyte, and it is a lithium secondary battery that does not include additive 2.

Comparative Example 2

It is a lithium secondary battery using an electrolyte solution further including additive 1 (1.0% by weight) and LiPO₂F₂ (0.5% by weight) as an additive.

Example 1

It is a lithium secondary battery using an electrolyte solution including additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

Example 2

It is a lithium secondary battery using an electrolyte solution further including additive 1 (1.0% by weight) and additive 2 (0.5% by weight).

Example 3

It is a lithium secondary battery using an electrolyte solution including additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

Example 4

It is a lithium secondary battery using an electrolyte solution including additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

For Comparative Examples 1 to 2 and Examples 1 to 3, the results of measuring the cell initial charging and discharging efficiency, the lifespan characteristics after 100 cycles of charging and discharging at high temperature (45° C.), and the rate-specific characteristics at a high rate (3C-rate) are shown in Tables 1, 2 and 3.

TABLE 1 Cell initial Additives (% by weight) efficiency Additive 1 LiPO₂F₂ Additive 2 (discharge/charge) <Comparative 1.0. — — 87.9% Example 1> <Comparative 1.0. 0.5. — 90.3% Example 2> <Example 1> 1.0. — 0.1. 90.6% <Example 2> 1.0. — 0.5. 90.1% <Example 3> 1.0. — 1.0. 90.2% <Example 4> 1.0. — 2.0. 87.8%

Table 1 shows the cell initial efficiencies for Comparative Examples and Examples, and the cell initial efficiency refers to a value obtained by dividing a discharge capacity by a charge capacity after charging once after the manufacturing of a lithium secondary battery is completed, discharging is performed, and then discharging is performed. The cut-off voltage was set to 2.5 V to 4.2 V, and the C-rate was tested at 1C at 45° C.

As a result of the experiment, in the case of Example 1 in which 0.1% by weight of additive 1 was added, it was confirmed that the initial cell efficiency was the best. Experimental results were found to be similar to those of Comparative Example 1 in Examples 2 to 4, and experimental results were similar to those of Comparative Example 2 in Examples 2 and 3.

A graph for this is shown in FIG. 2 .

TABLE 2 High- temperature Additives (% by weight) lifespan Additive 1 LiPO₂F₂ Additive 2 @100 cycle <Comparative 1.0. — — 88.9% Example 1> <Comparative 1.0. 0.5. — 90.8% Example 2> <Example 1> 1.0. — 0.1. 90.2% <Example 2> 1.0. — 0.5. 91.1% <Example 3> 1.0. — 1.0. 90.9% <Example 4> 1.0. — 2.0. 87.2%

Table 2 shows a high-temperature life for Comparative Examples and Examples and shows how much charging/discharging capacity may be maintained compared to an initial charging/discharging capacity after 100 cycles of charging and discharging are repeated. Similarly, the cut-off voltage was set to 2.5 V to 4.2 V, and the C-rate was 1C at 45° C.

As a result of the experiment, in the case of Example 2 in which 0.1% by weight of additive 2 was added, it was confirmed that the initial cell efficiency was the best. Examples 1 and 3 showed a high-temperature life similar to that of Comparative Example 2, and Example 4 showed experimental results lower than that of Comparative Example.

A graph for this is shown in FIG. 3 .

TABLE 3 Rate-specific Additives (% by weight) characteristics Additive 1 LiPO₂F₂ Additive 2 @3C-rate <Comparative 1.0. — — 73.6% Example 1> <Comparative 1.0. 0.5. — 76.7% Example 2> <Example 1> 1.0. — 0.1. 78.3% <Example 2> 1.0. — 0.5. 76.7% <Example 3> 1.0. — 1.0. 75.7% <Example 4> 1.0. — 2.0. 70.3%

Table 3 shows how much charge/discharge capacity can be maintained compared to the existing 1C-rate by three times increasing the rate speed compared to other experiments by measuring the rate-specific characteristics for Comparative Examples and Examples. Likewise, the cut-off voltage was set to 2.5 V to 4.2 V, and the experiment was performed at 45° C.

As a result of the experiment, in the case of Example 1 in which 0.1% by weight of the additive 1 was added, it was confirmed that the rate-specific characteristics were the most excellent.

A graph for this is shown in FIG. 4 . Through the experiments described above, when a lithium secondary battery is manufactured using an electrolyte using additives 1 and 2 at the same time, a lithium secondary battery with improved electrochemical properties can be obtained. This is presumed to be due to the fact that additive 2 strongly forms CEI and SEI at the cathode and anode.

From the experimental results, it was confirmed that additive 1 and additive 2 showed the highest electrochemical properties when added in a ratio of 10:1.

Although shown and described with respect to specific embodiments of the present disclosure, it is within the art that the present disclosure can be variously improved and changed without departing from the spirit of the present disclosure provided by the following claims. 

What is claimed is:
 1. An electrolyte solution for a lithium secondary battery, the electrolyte solution comprising an electrolyte salt and an organic solvent, wherein the electrolyte solution further comprises, as additives, vinylene carbonate (VC) represented by the following Formula 1, and a compound represented by the following Formula 2, (5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile):


2. The electrolyte solution of claim 1, wherein the 5-(4-nitrophenyl)-1-(3-((trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile is comprised in an amount of 0.1 to 2% by weight with respect to the total weight of the electrolyte solution.
 3. The electrolyte solution of claim 1, wherein the VC is comprised in an amount of 0.1 to 10% by weight with respect to the total weight of the electrolyte solution.
 4. The electrolyte solution of claim 1, wherein the 5-(4-nitrophenyl)-1-(trimethylsilyl)oxy)propyl)-1H-1,2,3-triazole-4-carbonitrile and the VC are added in a ratio of 1:10.
 5. The electrolyte solution of claim 1, wherein the electrolyte salt is any one or a mixture of two or more compounds selected from the group consisting of LiPF₆, LiBF₄, LiClO₄, LiCl, LiBr, LiI, LiB₁₀Cl₁₀, LiCF₃SO₃, LiCF_(3.0)CO₂, Li(CF₃SO₂)_(3.0)C, LiAsF₆, LiSbF₆, LiAlCl₄, LiCH₃SO₃, LiCF₃SO₃, LiN(SO_(2.0)C₂F₅)₂, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiB(C₆H₅)₄, and Li(SO₂F)₂N(LiFSI).
 6. The electrolyte solution of claim 1, wherein the electrolyte salt is comprised in a concentration range of 0.5 M to 1.0 M.
 7. The electrolyte solution of claim 1, wherein the organic solvent is any one or a mixture of ones selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.
 8. A lithium secondary battery comprising a cathode, an anode, a separator interposed between the cathode and the anode, and the electrolyte solution of claim
 1. 9. The lithium secondary battery of claim 8, wherein the cathode comprises a nickel-cobalt-manganese-based cathode active material.
 10. The lithium secondary battery of claim 9, wherein in the cathode active material, nickel, cobalt, and manganese are present in a ratio of 6:2:2 to 8:1:1. 